The present invention generally relates to fibrous sheets and more specifically to polymer additives for enhanced fibrous sheets.
Fibrous sheets are used for a variety of different purposes and are comprised of an array of different fibers, binders and fillers. For example, fibrous sheets can be used as acoustical ceiling tiles, paper products and furniture board. Primarily, fibrous sheets can be comprised of mineral wool, perlite, cellulosic fibers, fillers and binders.
Fibrous sheet production utilizes combinations of fibers, fillers, bulking agents, binders, water, surfactants and other additives mixed into a slurry and processed into a fibrous sheet. Examples of fibers used may include mineral fiber, fiberglass, and cellulosic material. Mineral wool is a lightweight, vitreous, silica-based material spun into a fibrous structure similar to fiberglass. Cellulosic material is typically in the form of newsprint. Added fillers may include expanded perlite, brighteners such as titanium oxide and clay. Expanded perlite reduces material density and clay enhances fire resistance. Examples of binders used in fibrous sheets include starch, latex and reconstituted paper products, which link together and create a binding system locking all ingredients into a structural matrix.
Organic binders, such as starch, are often the primary component providing structural adhesion for the fibrous sheet. Starch is often the preferred organic binder because it is relatively inexpensive. For example, fibrous sheets containing newsprint, mineral wool and perlite are often bound together by starch. Starch imparts both strength and durability to the fibrous sheet structure.
Unfortunately, there is a limit on how much starch can be added before such properties begin to decline. Starch is highly water-soluble and when partially hydrated loses a portion of its ability to bind the fibrous sheet components. Additionally, water-felted and cast panels tend to exhibit limited stability under high moisture loads given the hydrophilic nature of the cellulosic fibers. Furthermore, fibrous sheet strength and durability cannot simply be enhanced by using increased quantities of starch and cellulose, since starch increases a fibrous sheet""s susceptibility to moisture and sag.
Thus, a high degree of starch and cellulose can lead to sagging and weakening of the board under humid conditions. Also, fibrous sheets having large quantities of starch require elevated drying rates or increased drying time to remove excess water from the board. Therefore, there a need for a method for increasing both the strength and durability of a fibrous sheet without the addition of increased quantities of starch. Additionally, there is a need for a fibrous sheet that is not susceptible to sagging under high moisture loads and does not require increased drying times during processing.
The present invention encompasses both a method and composition for providing a two-part polymer binder additive for a fibrous sheet for improving both its strength and durability. The two-part polymer binder may be added to augment current organic binders to increase such desirable board properties as strength and durability or the polymers may be added to reduce the amount of organic binder required. Additionally, the polymers may be added in place of conventional organic binders or added in addition to organic binders to improve sag resistance and fibrous sheet performance.
The two-part polymer binder comprises both the addition of an anionic polymer having a negative charge of between about 4 to about 12 milliequivalents per gram and a cationic polymer having a positive charge of between about 6 to about 12 milliequivalents per gram. The two-part polymer binder forms an interpolyelectrolyte complex.
In greater detail, the two oppositely charged polymers act together to provide a synergistic effect that when added in addition with an organic binder, the strength and performance of the fibrous sheet is enhanced. The two oppositely charged polymers can coat and bond the organic binder to the fibers of the fibrous sheet to impart strength to the board.
Additionally, a surfactant may be added to the polymer binder. In a further embodiment a hydrogel and/or a latex may also be added. Combinations of all the described embodiments may also be used for the polymer binder.
The method of forming an enhanced fibrous sheet includes the steps of forming a fibrous slurry and mixing into the fibrous slurry an anionic polymer having a negative charge of between about 4 to about 12 milliequivalents per gram and then mixing a cationic polymer having a positive charge of between about 6 to about 12 milliequivalents per gram. The fibrous sheet is then dried to form the finished product.
Additionally, a fibrous sheet is provided comprising at least one type of fiber and an organic binder. The fibrous sheet also contains an anionic polymer having a negative charge of between about 4 to about 12 milliequivalents per gram and a cationic polymer having a positive charge of between about 6 to about 12 milliequivalents per gram.